The present invention relates to alternating current (AC) load power control in which AC power is supplied to a load under on-off control, the power actually applied to the load is detected, and the detected value is compared with the setting value to maintain the load power at a predetermined value.
In reproducing machines, for example, if the intensity of light emitted from an exposure lamp is not held constant, variations in the intensity of light will appear as a lack of uniformity in the copied image. Thus, it is important to accurately control the power applied to the load.
Such control is generally performed by phase control. In other words, an AC power source is employed to energize the load for a range of certain phase during each cycle of the power source, and the energizing phase is adjusted to control the load power. A generally used control system has a closed loop in which the voltage or current actually applied to the load is sampled to detect the actual load power, and the detected value is then compared with the setting value to determine a controlled amount (energizing phase angle).
Meanwhile, load power is changed attributable to variations in the source voltage. When attempting to phase-control the load power, those variations in the source voltage can be compensated to maintain the load power constant by detecting the source voltage and feeding back the detected information to the control process for correction of the energizing phase angle. In this case, however, if there is a time lag in the process of compensation control, the load power would change in such a period of lag time, and if the source voltage is erroneously detected, precise compensation would not be ensured.
Generally, in conventional feedback control, the detected value is compared with the target value and a minute value is added to or substracted from a control parameter dependent on the result of such comparison. Control of this type is performed satisfactorily in cases where the difference between the detected value and the target value is relatively small. But, in cases where that difference becomes larger due to abrupt changes the voltage source, the compensation process must be repeated many times to reduce that difference to zero and a longer period of time is required for complete compensation, during which time the load power may be continuously changed.
Furthermore, a conventional feedback control, the load voltage (or source voltage) is detected by calculating the average value with a CR integration circuit or microcomputer. In the system of this type, however, the form factor, peak factor, etc. would be changed if the waveform of the source voltage is not square, so that an error is enlarged which may occur in power actually applied to the load. Particularly, in the application fields, such as reproducing machines, where the large load is subjected to switching control (phase control), the waveform of the source voltage is largely different from that of a square wave and changed temporally in many cases. An error occurring in the detected data causes in turn an error in control, thus making it possible to maintain the load power constant.